The invention relates to ink jet printers having a plurality of continuous ink jets.
Apparatus has been developed over the past thirty years for direct printing onto receptor surfaces by emitting jets of ink drops from a print head under the control of information-carrying signals, to produce a record of the information (including both alpha numeric and graphical information) on the receptor surface. Such printers have developed mainly into two kinds, these being generally referred to as "continuous ink jet" and "drop on demand" printers respectively.
In continuous ink jet printers, the jets are emitted continuously, and selected ink drops are deflected from the stream of moving drops forming each jet, using a deflector responsive to the information-carrying signals. Printing can be effected by directing either the deflected or the undeflected drops towards the receptor surface, depending on the design of the particular printer being used. The remainder of the ink drops are generally caught, filtered and recycled. To enable the whole width of a page to be printed simultaneously, a multi-jet print head may be used having an array of such nozzles, each producing a continuous ink jet deflected independently of the others. The receptor surface can then be moved in a direction orthogonal to the array of nozzles in order to progress the printing along the receptor surface.
Deflection is usually effected by charging the drops and passing them through an electrostatic field, either the charge or the field being variable in response to the information-carrying signal. An early example of such a printer, using electrostatic deflection of a plurality of continuous jets is described by R G Sweet et al in U.S. Pat. No. 3,373,437. Other known forms of deflector include catcher tubes that are movable into or out from the flight path of the drops in response to information-carrying signals, but electrostatic deflectors are more usual.
To form a stream of drops, ink is conventionally forced under pressure through a fine nozzle, typically 25-80 .mu.m in diameter. The ink appears generally to emerge as a continuous ligament of liquid, which breaks up into a stream of separate drops spontaneously after a short distance, depending on nozzle size and flow rate. Uniformity can be assisted by piezoelectrical pulsing, but very small nozzles are still required. However this necessity for such fine nozzles has led to considerable practical problems in the past, in that they become totally or partially blocked very readily when any solid particles are present in the ink or when evaporation of the ink leads to encrustation at the tip. Inks formulated for such printers are desirably free from pigments; leading in turn to difficulties in achieving satisfactory optical density. This is a long recognized problem which has led to such proposals as the Hertz compound jet, originally proposed about ten years ago and described in U.S. Pat. No. 4,196,437; but the problems of nozzle clogging can still arise in modern commercial ink jet printers. Similar problems can also be caused by recycled ink, which requires careful filtering to remove solid pollutants picked up during each cycle (there being typically 40-50 cycles in printers recycling up to 98% of the drops ejected), the solids being picked up externally from the environment, or internally through precipitation due to solvent evaporation or pH changes or microbiological growth, or they may be deposited in the nozzles through drying out or chemical reaction during inactive periods, for example.
The procedures for drop-on-demand printers are very different in that the production of ink drops is intermittent, these only being produced when the print head is aligned with an appropriate part of the receptor surface, drop production being under the control of the information-carrying signals. Drops may be drawn from a row of nozzles when printing a whole width of page simultaneously, and although they do not have the dirt collecting problems of the recycled inks of the continuous jet printers, they are more prone to blocking of infrequently used nozzles through evaporation of the static exposed ink surface. IEEE Transactions on Industry Applications, Vol 1A-21, No 1, January/February 1985 describes how drops may be produced on demand from selected positions along a slot, instead of from individual nozzles, by providing a multiplicity of electrodes along the inside of the slot, one at each point from which release of a drop may be required. This slotted printing head is said to suffer less from clogging.